Sandia researchers have received a $1.2M award from the DOE’s SunShot Initiative to develop a technique that they believe will significantly improve photovoltaic (PV) materials efficiency and help make solar electricity cost-competitive with other energy sources.

Sandia researcher Vitalie Stavila inserts a substrate patterned with electrodes into a temperature-controlled liquid-phase reactor for depositing MOF thin films. Sandia’s research team plans to combine MOFs with dye-sensitized solar cells, a technique it believes will lead to advancements in photovoltaic technology. (Photo by Dino Vournas)

The work builds on Sandia’s recent successes with metal-organic framework (MOF) materials by combining them with dye-sensitized solar cells (DSSCs). DSSCs, invented in the 1980s, use dyes designed to efficiently absorb light in the solar spectrum. The dye is mated with a semiconductor, typically titanium dioxide, that facilitates converting the energy in the optically excited dye into usable electric current. DSSCs are considered a significant PV technology advancement because they separate the various current generating processes from a solar cell.

“A lot of people are working with DSSCs, but we think our expertise with MOFs gives us a tool that others don’t have,” said Erik Spoerke (in Sandia’s Electronic, Optical, & Nano Materials Dept.), a materials scientist with a long history of solar-cell exploration at the Labs.

“If you don’t have everything in the DSSC dependent on everything else, it’s a lot easier to optimize your PV device in the most flexible and effective way,” explained Sandia senior scientist Mark Allendorf (in Sandia’s Transportation Energy Center). DSSCs, for example, can capture more of the sun’s energy than silicon-based solar cells by using varied or multiple dyes and also can use different molecular systems, Allendorf said.

In experiments published in Science late last year, Sandia researchers added a molecule known as TCNQ to a metal-organic framework (MOF) that creates an electrically conducting path through the MOF structure. The turquoise spheres represent carbon atoms, white are hydrogen, blue are nitrogen, red are oxygen and bronze are copper.

“It becomes almost modular in terms of the cell’s components, all of which contribute to making electricity out of sunlight more efficiently,” said Spoerke.

Though a source of optimism for the solar research community, DSSCs pose certain challenges that the Sandia research team thinks can be overcome by combining them with MOFs. MOFs are crystalline and highly-ordered materials that also offer high levels of porosity, said Allendorf, a MOF expert and 29-year Sandia veteran. He calls the materials “tinkertoys for chemists” because of the ease with which new structures can be envisioned and assembled. MOF’s unique porosity will allow researchers to add a second dye, placed into the MOF pores, that will cover additional parts of the solar spectrum that weren’t covered with the initial dye.

Allendorf and Spoerke are convinced that MOF’s ordered structure and versatile chemistry can help improve DSSC overall electron charge and flow, which currently faces instability issues. “Essentially, we believe MOFs can help to more effectively organize the electronic- and nano-structure of the molecules in the solar cell,” said Spoerke.

In addition to the Sandia team, the project includes researchers at the University of Colorado-Boulder, particularly Steve George, an expert in a thin-film technology known as atomic layer deposition. “With the combination of MOFs, DSSCs, and atomic layer deposition, we think we can figure out how to control all of the key cell interfaces and material elements in a way that’s never been done before,” said Spoerke. “That’s what makes this project exciting.”

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.